Myothermic, polarographic, and fluorometric data from mammalian muscles: correlations and an approach to a biochemical synthesis.

Energy output of mammalian cardiac, smooth, and skeletal muscles has been studied by recording oxygen consumption, heat production, the fluorescence of reduced nicotinamide adenine dinucleotide (NADH), and lactate production. Active oxygen consumption and heat production of all muscle types correlate well if one assumes a calorific equivalent of 20.2 kJ/liter of oxygen. At 27 C aerobic lactate production is low in all muscle types and is not appreciably increased after mechanical activity. Early oxidation of NADH within the time course of the mechanical event suggests that no temporal basis exists for distinguishing between initial and recovery heat production in mammalian muscles. For muscles consuming energy at low rates, the time course of fluorescence recovery to baseline correlates well with the time course of late heat production, and the area enclosed by a fluorescence waveform (fluorescence-time integral) correlates well with total energy consumption. At high energy rates these correlations disappear as the fluorescence waveforms display complex kinetics. A digital computer simulation of oxidative phosphorylation and intermediary metabolism predicts failure of correlation between mitochondrial NADH levels and other metabolic indices under such conditions. This lends indirect support to the assumption that cytoplasmic NADH does not complicate the fluorescence signals recorded from aerobic unpoisoned muscles.